var nav = document.querySelectorAll(".nav > li");
var main = document.querySelectorAll(".main > div");

for(var i = 0; i<nav.length; i++){
    nav[i].index = i;
    nav[i].addEventListener('mouseover',function() {
        for (var j = 0; j < nav.length; j++) {
            nav[j].classList.remove('active');
            main[j].classList.remove('active');
        }
        console.log(this);
        this.classList.add('active');
        console.log(this.index);
        main[this.index].classList.add('active');
    })
}
